Water cooling towers are in widespread use for large capacity heat exchange systems. These cooling towers are used to remove absorbed heat from a circulating water coolant by evaporating a portion of the coolant in the cooling tower. The remaining coolant is extracted from a reservoir or sump at the base of the tower by a pump, and supplied through the heat load on a continuous basis. Because a large quantity of water evaporates in such a system, a significant and fairly rapid build-up of scale, sludge and the like takes place in the water. Various chemicals are employed for preventing the precipitation out of the minerals in the water, since such precipitation causes what is known as "scaling" on the surfaces of the cooling tower and heat exchange equipment. If such scaling is not prevented or is not periodically removed, it can result in significantly reduced heat transfer, and, therefore, significantly increased operating costs. In addition, it enhances corrosion of the heat exchange surfaces beneath the scale.
In addition to the scaling problem mentioned above, if the pH factor in the water is too low (typically, below 6.8), increased corrosion of the system components can result. If the pH is too high, the significant scale build-up mentioned above, takes place. Consequently, it has been the practice to add chemicals to the cooling tower water to maintain a balanced or "safe" range of pH factor in the coolant.
Even with the addition of chemical additives to maintain a chemical balance in the coolant water, the constant significant evaporation of a portion of the water rapidly builds up the amount of total dissolved solids (TDS) in the water to the point where scaling can occur, even though scale-retardant chemicals are present in the water. To prevent scaling from occurring, it is customary periodically to remove water with a high TDS content from the system to reduce the TDS, or to increase the oxidizing potential or pH. When water is removed, it is replaced with "makeup" water, which typically is obtained from the local available or modified water supply. The removal or dumping of water from the system is accomplished in one or the other of two ways, namely a "bleed-off" in which a portion of the sump or reservoir water is drained while the system is operating, or by "blow-down", which typically is a complete draining of the sump. In both cases, the water which is drained off is replaced with the "make-up" water.
Typically, the dumping of water by either "bleed-off" or "blow-down" in cooling tower systems, is effected when the TDS is somewhere around 5,000 ppm. In most systems, a TDS in excess of 5,000 ppm results in scaling, even with the use of chemical additives and oxidants.
In the past, the decision to initiate dumping of water from the reservoir of the cooling tower generally has been manually determined. In some cases, such dumping simply is done on a periodic basis. In other cases, a visual inspection of the water in the cooling tower is made to determine whether or not to effect dumping of some of that water. This is a very imprecise technique, and typically results in the dumping of excessive quantities of water from the system.
A system, which has been developed for providing a somewhat more precise control of the dumping or bleed-off of water from a cooling tower system, is disclosed in the patent to O'Leary U.S. Pat. No. 4,464,315. This patent includes probes for sensing the conductivity of the cooling tower water, as well as the conductivity of the make-up water. The probes provide signals to a controller unit, which proportionally adjusts the trip point at which dumping of the cooling tower water occurs, based upon the sensed water conductivities. A problem with a system of the type disclosed in the O'Leary patent, however, is that at TDS concentrations of 5,000 ppm or less, the differences in conductivity at different TDS concentrations are relatively small; so that accurate determination of the concentration required for dumping of water is difficult. In addition, for such low TDS concentrations, frequent dumping, either by way of blow-down or bleed-off, is required, resulting in the waste of a substantial quantity of water.
The water which is dumped from a cooling tower system represents a significant loss. In a typical, relatively small and relatively efficient evaporative cooling tower system, a total amount of approximately 637,000 gallons per year of water is consumed by the cooling tower. Of this amount, 437,000 gallons are evaporated in the cooling process. The other 200,000 gallons is flushed down the sewer in the bleed-off operation. This ratio, of approximately one-third of the total amount of water used being totally wasted, is typical of such systems. Some cooling tower systems waste an even greater quantity of water. Not only does this wasted water result in a appreciable loss, both in the resource of the water itself, and in its cost; but most cooling tower systems use chemicals or additives, such as acids, anti-fouling agents and corrosion inhibitors, to prevent corrosion and scaling from occurring. These chemicals constitute a substantial environmental risk, since in many cases the chemicals are toxic and/or hazardous.
The patent to Derham U.S. Pat. No. 4,931,187 discloses another cooling tower system in an effort to automate control of pH, temperature and TDS. In the Derham system, these parameters are monitored to control the addition of make-up water to the cooling tower, either by means of a bypass or through a water softener, or both, in accordance with the different variables measured by the system. Derham states that the system eliminates virtually any bleed-off of coolant. To accomplish this, however, an additional particle filter must be included to remove suspended solids from the coolant. It is necessary, however, to backwash the filter with water from outside the system on a periodic basis. It is not clear how scaling can be prevented without any dumping of the water recirculated in the cooling tower system, since at some point the TDS in that water reaches a saturation point. While some precipitated solids will be removed by the filter 20, it appears that scaling also can occur as a result of saturation, irrespective of the operation of the water softener and the filter. These additional components also require extra periodic maintenance for their operation.
Other cooling tower systems have been designed, which include an automatic pre-established timer control of backwash and chemical addition (Derham U.S. Pat. No. 3,628,663) or some type of float responsive to the water level in the cooling tower for controlling either make-up water addition (Kinkead U.S. Pat. No. 4,836,239), or make-up water addition and chemical addition (O'Leary U.S. Pat. No. 3,788,340 and Glad U.S. Pat. No. 3,627,032).
In addition to corrosion and scaling conditions, materials are introduced through air, water and environmental changes, which provide sources for biological support of aerobic or anaerobic algae and the like. While, to some extent, suspended solids in the water may provide a visual indication of the existence of undesirable biological conditions, typically, the conditions are not discernable to the viewer until a critical condition exists.
It is desirable to provide automatic control of the electron equilibrium of the water in a system, to significantly reduce the amount of water which must be dumped from the system, to accurately sense the conditions required for the addition of chemical additives, with a minimum of maintenance or supervision in its operation.